</font><font color="blue" class="small">( Just noticed on my TSC brand 4' bush hog. Looks like a 1/2" bolt. There is nothing else in the link that will shear. It must be a perfect because there is no slop. )</font>
If the implement end of the PTO shaft slips over a round transmission shaft with a hole through it, then you use a shear bolt the same size as the hole. If the transmission shaft is splined, then it's simply a fastening bolt. The shear bolt principle only works on smooth shafts.
Assuming it is in fact a shear bolt, Grade 2 hardness is usually specified. You must get the right diameter, or the hole will become egg-shapped. The length need be no longer than necessary to thread a self-locking nut until it's flush.
//greg//